Automatic flight control method for a rotorcraft enabling the rotorcraft to maintain a path by tracking manual flight controls

ABSTRACT

A method of enabling an autopilot ( 9 ) to cause a rotorcraft ( 1 ) to follow a path. At least one guide mode (G) relative to at least one progression axis (P, R, V, Y) of the rotorcraft ( 1 ) is selected by the rotorcraft pilot. Said selection causes the selected guide mode (G) to be inhibited ( 19 ) and causes a path setpoint (C) to be acquired ( 20 ) from the pilot of the rotorcraft ( 1 ) operating a manual control member ( 4 ) for controlling the progression of the rotorcraft ( 1 ). The path setpoints (C) relating to other guide modes (G) of the rotorcraft ( 1 ) that continue to be engaged are conserved in their initial states and the autopilot ( 9 ) adapts the commands relating to the progression axes (P, R, V, Y) relating to these other guide modes (G).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/908,364, filed Jun. 3, 2013, which claims priority to French patentapplication No. FR 12 01620 filed on Jun. 6, 2012, the disclosures ofwhich are incorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the field of rotorcraft, and moreparticularly to automatic flight control systems that have an influenceon the flight behavior of the rotorcraft. The automatic flight controlsystem for a rotorcraft to which the present invention relates is anautomatic system for providing the pilot of the rotorcraft withassistance in performing difficult procedures acting on the flightbehavior of the rotorcraft, in particular at low speeds, such as speedsof less than about 50 knots (kts).

(2) Description of Related Art

Rotorcrafts are aircrafts that differ from other powered aircraft mainlyin their ability to fly both at high cruising speeds and to fly at lowspeeds or to hover. This ability is obtained by providing the rotorcraftwith at least one main rotor having an axis that is substantiallyparallel to the anteroposterior plane of the rotorcraft and to theinstant trim plane of the rotorcraft.

The main rotor comprises a rotary wing that provides the rotorcraft withlift and possibly also with thrust. The behavior of the rotorcraft inflight is modified by varying the cyclic and/or collective pitch of theblades of the rotary wing. A variation in the cyclic pitch of the bladesleads to a change in the attitude of the rotorcraft, and moreparticularly to a change that can be varied between pitching and/orrolling. A variation in the collective pitch of the blades gives rise toa modification to the lift behavior of the rotorcraft, and moreparticularly behavior along the gravity axis or the instant trim plane.

When the main rotor provides lift only, and possibly also with marginalpropulsion as happens with a hybrid helicopter, the rotorcraft is fittedwith means specifically for providing propulsion. For example, hybridhelicopters have a main rotor serving to provide the major portion ofthe lift of the rotorcraft and, to a lesser extent, its propulsion,together with at least one propulsive propeller.

The rotorcraft is also maneuverable in yaw about its own vertical axis,by making use of a yaw anti-torque device. For example, such ananti-torque device is formed by an anti-torque rotor having an axis thatis substantially horizontal compared with the substantially verticalaxis of the main rotor. By way of example, the anti-torque rotor is atail rotor of the rotorcraft, or, by way of example and in a hybridhelicopter, it is formed by at least one of the propulsive propellersfitted to such a hybrid helicopter. With an anti-torque rotor, varyingthe collective pitch of the blades of the anti-torque rotor gives riseto a variation in the yaw progression of the rotorcraft.

The specific ability of a rotorcraft to fly at low speeds or to hoverimplies a special organization for the means used to manage its flightbehavior. The difficulties associated with such a special organizationneed to be considered independently of potential use of said means forhigh-speed progression of the rotorcraft.

A variation in the flight behavior of the rotorcraft is achieved bymodifying flight parameters of the rotorcraft, which parameters aredefined relative to cyclic and/or collective pitch values for the mainrotor and/or collective pitch values for the anti-torque rotor. Such amodification to the flight parameters may be achieved using variouscontrol modes.

In a manual control mode, the pilot of the rotorcraft has manual controlmembers that are moved by a person in order to vary the pitch of theblades or of the rotors by means of manual control linkages respectivelyallocated to each of the progression axes of the rotorcraft. The conceptof “manual” should be understood as being in opposition to the conceptof “automatic”, without prejudice to the means actually used by a personfor controlling the rotorcraft, in particular a hand-operated stick orfoot-operated rudder pedals.

In an embodiment of a manual control mode, the manual control membersare engaged with respective linkages for transmitting forces remotely,enabling the pilot of the rotorcraft to act mechanically on the bladesby means of manual control members, either directly or else viaservo-controls for a heavy helicopter.

In another embodiment of a manual control mode, movement of a manualcontrol member by the pilot generates electrical signals for activatingat least one blade-moving servo-control.

In an automatic control mode, an autopilot generates commandsautomatically for moving the blades by means of automatic controllinkages allocated respectively to each of the progression axes of therotorcraft. When the autopilot is activated, the automatic commands takethe place of the commands generated by the pilot directly on the manualcontrol members in order to activate the servo-controls.

The autopilot serves to maintain stable progression of the rotorcraft incompliance with a previously-stored setpoint. An actual state ofprogression of the rotorcraft is evaluated by the autopilot at a giveninstant, according to information supplied by onboard instrumentation ofthe rotorcraft. On the autopilot detecting a difference between thesetpoint and the actual state of progression of the rotorcraft, theautopilot intervenes on the flight behavior of the rotorcraft in orderto bring its actual state of progression back into conformity with thesetpoint.

Activation of the autopilot is controlled by the pilot of the rotorcraftusing one or more specific control knobs.

In a stabilization mode performed by the autopilot, an initial setpointfor maintaining the attitude of the rotorcraft is defined relative tothe state of progression of the rotorcraft as evaluated on activatingthe autopilot. The stabilization mode stabilizes the rotorcraft bycorrecting the attitude of the rotorcraft by means of the autopilotacting relative to the initial setpoint.

On this topic, reference may be made for example to document FR 1 347243 (Boeing Co.), which discloses ways of stabilizing the behavior of arotorcraft by an autopilot by holding to previously-stored flightsetpoints. The detection of a deviation of the rotorcraft from its pathgenerated error signals, and flight commands are issued by the autopilotto correct the attitude of the rotorcraft until the error signals arereduced to a value of zero. The flight commands are issued by theautopilot for at least one of the progression axes of the rotorcraft ina manner that is mutually synchronized for all of the progression axesso that the behavior of the rotorcraft is kept stable in the event ofthe position of the rotorcraft being corrected relative to any one ofthe progression axes.

In a particular mode of piloting by transparency, the pilot of therotorcraft may optionally act temporarily on the behavior of therotorcraft by using the manual control linkages, thereby overruling thecommands generated by the autopilot. The initial setpoint is leftunchanged, and any temporary action on the part of the pilot on thebehavior of the rotorcraft does not lead to any modification of theinitial setpoint.

It is also known to correct an initial setpoint for maintaining attitudeas a function of the actual state of progression of the rotorcraft asevaluated after the pilot has operated the manual control members. It isalso known to enable the pilot of the rotorcraft to correct an initialsetpoint for maintaining attitude by varying its values incrementally.

On this topic, reference may be made for example to the document GB 809278 (Bendix Aviat Corp.), which describes such ways of correctingsetpoints for maintaining attitude. More particularly, an autopilotmaintains the positions of the elevators and the ailerons of an airplanein compliance with flight setpoints to achieve progression of theairplane that is stabilized in pitching, in roll, and in yaw. A humanpilot can act on the progression of the airplane by modifying thepositions of the ailerons, while conserving autopilot stabilization ofthe behavior of the airplane. At the end of the action taken by thehuman pilot, and once the attitude of the airplane is stabilized at thedesired altitude, the flight setpoints are conserved in the currentstate of progression of the rotorcraft.

Stabilization of the rotorcraft is achieved using basic modes in whichthe autopilot acts e.g. to generate increased stability by dampingangular movements of the rotorcraft, or indeed to maintain attitudes ora heading, or indeed to decouple progression axes, for example. Thesebasic modes provide piloting comfort for the pilot of the rotorcraft,but they do not correct possible deviations of position of therotorcraft. Proposals have therefore been made to associate higher modesof operation with such basic modes in order to eliminate possibledeviations of position, of speed, and/or of acceleration of therotorcraft. The behavior of the rotorcraft is managed by the autopilotas a function of the flight setpoint so as to keep the rotorcraft stableand so as to reestablish its position, its speed, or its acceleration byusing the higher modes. The autopilot performs the operation ofstabilizing the rotorcraft quickly by using the basic mode, whereas itsubsequently performs the operation of reestablishing the position, thespeed, and/or the acceleration of the rotorcraft more slowly by usingthe higher modes.

The autopilot is also capable of performing advanced functions ofassisting the guidance of the rotorcraft. The facilities potentiallymade available by the higher modes are used in auxiliary manner toachieve such assistance. Various advanced functions may be used toachieve assistance in the guidance of the rotorcraft. The ways in whichadvanced functions are performed relate to predefined functionalities ofthe autopilot relating to a path to be followed by the rotorcraft.

In various advanced functions of the autopilot making use of the highermodes, the rotorcraft is guided by the autopilot relative to apreviously-defined setpoint path. The autopilot can then make use ofvarious geolocation means for guiding the rotorcraft along a setpointpath.

By way of example, the setpoint path is used relative to a flightmission as previously determined by the pilot of the rotorcraft, or itis used during a stage of approaching a known and identified site. Inparticular, such a site is fitted with means that provide interactionbetween the site and the autopilot, such as radio navigation beacons. Inthe absence of such interactive equipment, site identification isperformed by the pilot of the rotorcraft in manual mode, and then thepilot of the rotorcraft activates the desired advanced function.

For such advanced functions making use of the higher modes of theautopilot, two superposed loops are used for servo-controlling flightparameters. A fast servo-control loop is used for correcting attitude,yaw, or verticality of the rotorcraft. A slow servo-control loop is usedby the higher mode for reducing any guidance deviation of the rotorcraftto zero.

The autopilot is conventionally in communication with display means thatprovide the pilot of the rotorcraft with various kinds of information,e.g. such as information about various flight parameters, about theflight mission to be performed by the rotorcraft, about weatherconditions, and/or about the environment outside the rotorcraft, orindeed about the environment at the site of intervention. Suchinformation is useful for controlling the rotorcraft by the autopilotand/or by the pilot of the rotorcraft.

On this topic, reference may be made for example to the document US2011/137492 (Sahasrabudhe Vineet et al.), which discloses ways ofautomatically guiding a rotorcraft along a path by making use of saidadvanced functions (a vertical takeoff and landing (VTOL) function).Guidance of the rotorcraft is performed in flight using instruments,while taking account of an actual state of progression of the rotorcraftand a state of progression that the rotorcraft is to achieve so as to beguided along a predefined approach path. The human pilot has informationdisplay means available that display information about guiding therotorcraft along the path relative to the outside environment.

The ways in which the autopilot operates provide automatic assistance topiloting that is satisfactory in terms of correcting the attitude of therotorcraft in a cruising stage of flight, at high speed, and while therotorcraft is far away from the ground. During a cruising stage offlight, the surroundings of the rotorcraft are normally empty, and thepilot of the rotorcraft does not need to concentrate on maneuvering therotorcraft. It can also happen that there is no need for suchconcentration when close to the ground and in a known environment, withthis being made possible by using an advanced function of the autopilot,such as during a stage of approaching a known runway and/or a runwaythat is fitted with means for identifying its environment.

Automatic assistance obtained by the autopilot performing an advancedfunction can be satisfactory during a stage of approaching anintervention site, including at low speed, providing the interventionsite is well known, identified, and indicated to the autopilot. Once theintervention site has been identified, it is possible to activate anadvanced function in order to guide the rotorcraft along thecorresponding setpoint path.

In general, rotorcraft are powered aircraft that are designed to be usedunder flight conditions that are difficult, such as at low speeds orwhile hovering, close to the ground anywhere, in a position that may beunknown or poorly known, and with arbitrary conditions of visibilityand/or an environment that is hostile and/or unknown.

Under difficult flying conditions, unexpected factors might need to betaken into account by the pilot of the rotorcraft. It is difficult oreven impossible for the pilot of the rotorcraft to make use of automaticassistance in maneuvering the rotorcraft under such difficultconditions. For example, when the rotorcraft is close to the ground, anychange needed in its behavior must be performed quickly. The ways inwhich the autopilot operates make it difficult to act quickly to modifya path to be followed by the rotorcraft by making use of an advancedfunction that implements the higher modes.

Thus, a landing zone might be poorly known while the pilot of therotorcraft is preparing a flight mission. Access conditions to thelanding zone might initially be identified as being potentiallydifficult, or unknown, or indeed hostile. Access to the landing zone mayalso be made particularly difficult, since the real environment at thesite might be different from and/or temporarily modified relative to theexpectations of the pilot of the rotorcraft. Prior location of thelanding zone using geolocation means can be approximate, and possiblyeven uncertain. Poor visibility does not make it any easier for thepilot of the rotorcraft to identify quickly on site the difficultiesthat need to be overcome in order to approach the landing zone.

Under such conditions, the pilot of the rotorcraft can become confusedas to which automatic or manual control modes should be selected inorder to approach a landing zone in a difficult flying situation. It isthen found that the pilot of the rotorcraft needs to have available theadvantages provided by piloting assisted by means of the autopilot,while still retaining the ability to intervene quickly in manual mode onthe behavior of the rotorcraft.

It is useful to avoid the pilot of a rotorcraft becoming confused inthis way during a difficult stage of approach to an intervention site,by enhancing the ways in which the autopilot operates so as to make itpossible for the human pilot to intervene quickly on the behavior of therotorcraft.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of using anautomatic flight control system for a rotorcraft that enables arotorcraft pilot to be relieved of such a feeling of confusion during adifficult approach stage to an intervention site.

The method of the invention is a method of using an automatic flightcontrol system for a rotorcraft. Said automatic system has an autopilotthat generates commands in predefined operating modes. The commandscause the behavior of the rotorcraft to be modified relative to at leastone of the progression axes of the rotorcraft comprising at least thepitching, roll, vertical, and yaw axes, and possibly also the horizontalaxis for a hybrid helicopter having propulsive propellers. The term“progression axes” is used to designate the control axes relative towhich the progression of the rotorcraft in flight is controlled,regardless of whether that is under the control of the pilot of therotorcraft and/or of the autopilot. On the basis of a commandimplemented relative to said progression axes, the behavior of therotorcraft is modified relative to its six axes of freedom in theterrestrial frame of reference.

The said automatic control system is commonly in communication withonboard instrumentation of the rotorcraft, thus making it possible toevaluate the current state of progression of the rotorcraft at a giveninstant. On the basis of such an evaluation, the autopilot isadvantageously suitable for acting on the flight behavior of therotorcraft on the basis of previously-stored flight setpoints.

In the present invention, the method is mainly recognizable in that theautopilot incorporates a path-maintaining function for the rotorcraft.Said path-maintaining function comprises at least one guide mode of theautopilot for guiding the rotorcraft relative to at least one of saidprogression axes of the rotorcraft.

More particularly, in essential ways of performing the path-maintainingfunction, an acquisition request for acquiring at least one pathsetpoint defined by at least one flight parameter is issued by a person.Said path setpoint relates to said at least one guide mode. Thereafter,as a consequence of the person moving at least one manual control memberto cause a modification to the behavior of the rotorcraft relative to atleast any one of said progression axes, the autopilot generates:

an inhibit command whereby the autopilot inhibits said at least oneguide mode of the rotorcraft for which said acquisition request toacquire a path setpoint has been issued and relating to said at leastone progression axis relative to which the behavior of the rotorcraft isbeing modified by the person; and

an acquisition command whereby the autopilot acquires the value of saidat least one flight parameter relating to said path setpoint to beacquired and applicable to said at least one progression axis from whichthe behavior of the rotorcraft is being modified by the person, saidvalue being evaluated under the condition said movement applied to themanual control member being interrupted. Said value on its own takes theplace of the value of at least one said flight parameter acquired by theautopilot prior to the inhibit command.

It should be understood that said acquisition command relates to said atleast one flight parameter concerning said path setpoint to be acquired.Under the effect of said person interrupting movement applied to themanual control member, the path setpoints relating to the other guidemodes of the rotorcraft that have continued to be engaged and thatrelate to said other progression axes are conserved in the state theyhad prior to said person moving said at least one manual control member.

The commands generated by the autopilot relating to all of theprogression axes that continue to be engaged are adapted by theautopilot depending on the current state of progression of therotorcraft on the basis of said conserved path setpoints and on thebasis of said value taking the place of said at least one flightparameter for which said acquisition request was issued.

The value of said flight parameter relating to the path setpoint to beacquired is evaluated in particular depending on the actual state ofprogression of the rotorcraft as determined from the onboardinstrumentation of the rotorcraft. The value of said flight parameterrelating to the path setpoint to be acquired is evaluated at the timethe person interrupts movement of the manual control member, where thatmovement is what generated a modification in the behavior of therotorcraft relative to the path setpoint for which the acquisitionrequest was issued by the person.

It may be observed that the autopilot may acquire the value of anyflight parameter before the inhibit command was issued by using anyacquisition means, such as means commonly used in the field ofrotorcraft. In other words, the way in which the initial value of aflight parameter is acquired before the inhibit command is immaterialwhen considering the way in which the value of this flight parameter isacquired depending on the specific ways in which the path-maintainingfunction of the present invention is implemented.

In terms of guiding the rotorcraft, the concept of “other” progressionaxes should be considered as designating the remainder of the set ofsaid progression axes of the rotorcraft after said at least oneprogression axis of the rotorcraft relating to the guide modeconstituting the subject of said inhibit command has been excluded fromsaid set. Said other guide modes continue to be engaged, and should beconsidered as being guide modes for which said acquisition request didnot generate a said inhibit command.

This is to be distinguished from a basic mode of the autopilot thatserves to stabilize the rotorcraft, or indeed from decouplingprogression axes of the rotorcraft, and a said guide mode of therotorcraft. Such a basic mode can be performed or inhibited at the sametime as the inhibit command is issued for inhibiting said at least oneguide mode of the rotorcraft.

More particularly, in an implementation of the method, the inhibitcommand for inhibiting said at least one guide mode of the rotorcraft ispreferably associated with a command for maintaining a basic mode of theautopilot for stabilizing the rotorcraft by damping its attitude.

The request for acquiring the path setpoint includes in particular aselection operation performed by a person using at least one said guidemode. Such a selection operation is performed by the person acting on amultiple choice control member. By using such a multiple choice controlmember, the person selects at least one said guide mode from any one ofat least a plurality of guide modes. Such a plurality of guide modesadvantageously relate respectively at least to:

maintaining a rate of progression of the rotorcraft in attitude. Such aguide mode is preferably associated with the said command formaintaining a mode in which the rotorcraft is stabilized by theautopilot;

maintaining a rate of progression of the rotorcraft vertically;

maintaining a hovering position;

maintaining a rate of progression of the rotorcraft in yaw;

maintaining an orientation of the rotorcraft relative to a heading;

maintaining a slope of the rotorcraft;

maintaining an acceleration of the rotorcraft relative to the axes ofthe local geographical frame of reference;

maintaining an acceleration of the rotorcraft relative to theprogression axes of the rotorcraft;

maintaining a coordinated turn by the rotorcraft;

maintaining the position of the rotorcraft relative to side slip;

maintaining an air speed of the rotorcraft;

maintaining an angle of incidence of the rotorcraft;

maintaining an altitude of the rotorcraft;

maintaining a height of the rotorcraft; and

maintaining a speed of the rotorcraft along a path, or analogously aground speed.

The vertical rate of progression acquired by the autopilot is likely tobe zero, in order to maintain the center of gravity of the rotorcraft ina given vertical position, regardless of whether it is measured in termsof altitude or of height. Such a zero vertical rate of progression mayoptionally correspond to maintaining the rotorcraft in hovering flight.

The vertical progression of the rotorcraft may correspond equally wellto the rotorcraft moving away from or towards a landing zone. Such aguide mode corresponds in particular to the autopilot acquiring a pathsetpoint based on maintaining a vertical position or a vertical positionvariation for the rotorcraft in order to assist the pilot of therotorcraft by performing a path-maintaining function relating tomaintaining a slope. Such a position or position variation is maintainedin particular by acquiring a collective pitch value for the blades ofthe main rotor and possibly by adapting the current values of the cyclicpitch of the blades of the main rotor of the rotorcraft in pitchingand/or in roll, and the collective pitch of the blades of an anti-torquerotor or analogous values relating to using any anti-torque device.

In particular, the orientation of the rotorcraft relative to a headingmay be maintained equally well relative to geographic north or tomagnetic north. Such a guide mode corresponds in particular to theautopilot acquiring a path setpoint based on maintaining the orientationof the rotorcraft in yaw, such as from acquiring a collective pitchvalue for the anti-torque rotor or the like, and the autopilot adaptingthe current values of the cyclic pitch and of the collective pitch ofthe blades of the main rotor.

According to a general feature of the present invention, it should beconsidered that the guide mode relating to maintaining the orientationof the rotorcraft relative to the yaw axis may be performed using anyanti-torque device, e.g. such as an anti-torque rotor, a tail rotor, ora propulsive propeller of a hybrid helicopter, in particular, or an airjet device, or any other analogous anti-torque device. The arrangementdescribed relating to using such a tail rotor may be transposed, whereappropriate, to the specific modes of operation of any other anti-torquedevice of different structure that might be used.

The guide mode for maintaining the slope of the rotorcraft may applyequally well to tracking a ground slope or an air slope. Such a guidemode corresponds for example to the autopilot acquiring a path setpointbased on maintaining an angle of inclination of the rotorcraft, whichmay equally well be in pitching and/or in roll, and varying the verticalposition of the rotorcraft. Such provisions advantageously lead topiloting assistance for the purpose of moving the rotorcraft towards, orconversely away from, a predetermined landing zone, with a givenattitude orientation of the rotorcraft. Maintaining an inclination angleof the rotorcraft in this way can potentially be obtained by acquiringcyclic pitch values for the main rotor, which may equally well be inpitching and/or in roll, by adapting the current value of the collectivepitch of the anti-torque rotor, and equally well by adapting the currentvalue or acquiring the value of the collective pitch of the blades ofthe main rotor.

Maintaining the acceleration of the rotorcraft may be considered equallywell as being positive or negative. Such a guide mode corresponds inparticular to the autopilot acquiring a path setpoint based onmaintaining a forward direction of the rotorcraft as defined relative tothe pitching, roll, yaw, and vertical axes, while taking account of avariation in the speed of the rotorcraft along at least any one of saidprogression axes.

Depending on the at least one progression axis of the rotorcraft takeninto consideration at the time the corresponding manual control memberis moved, such a variation in the speed of the rotorcraft may beobtained for example by acquiring values for a continuous variation inthe pitch of the blades of the main rotor, which may apply equally wellvarying cyclic pitch and/or varying collective pitch. Also by way ofexample, said speed variation may be procured by continuously varyingthe collective pitch of the blades of the anti-torque rotor. The currentvalues for the pitch of the blades relating respectively to one or moreof the progression axes of the rotorcraft that have not been the subjectof a person moving a corresponding manual control member are adaptedaccordingly.

In general, the acquisition by the autopilot of said path setpointrelating to said acquisition command is in particular an acquisition:

either of a flight parameter relating to a variation in the collectivepitch of the blades of a main rotor of the rotorcraft, which parametermay be a constant value or a value variation to be maintained. The valueof a flight parameter relating to a variation in the cyclic pitch of themain rotor and the value of a flight parameter relating to a variationin the collective pitch of an anti-torque rotor of the rotorcraft remainunchanged;

or a flight parameter relating to a variation in the cyclic pitch of theblades of the main rotor, being either a constant value or a valuevariation to be maintained. The value of a flight parameter relating toa variation in the collective pitch of the blades of the main rotor andthe value of a flight parameter relating to a variation in thecollective pitch of the anti-torque rotor remain unchanged;

or both of said flight parameters relating to respectively to acollective pitch variation and to a cyclic pitch variation of the bladesof the main rotor, which may be constant values or value variations tobe maintained. The value of a flight parameter relating to a variationin the collective pitch of the blades of the anti-torque rotor remainsunchanged;

or else a flight parameter relating to a variation of the collectivepitch of the blades of the anti-torque rotor, which may be a constantvalue or a value variation to be maintained. The value of a flightparameter relating to a variation in the cyclic pitch of the main rotorand the value of a flight parameter relating to a variation in thecollective pitch of the main rotor remain unchanged.

The value of said at least one flight parameter is evaluated dependingon the actual state of progression of the rotorcraft as determined byonboard instrumentation of the rotorcraft. Such onboard instrumentationcomprises a plurality of measurement means that are commonly fitted torotorcraft. By way of indication and as a non-exhaustive enumeration,such measurement means may comprise:

means for determining a rate of progression of the rotorcraft along anyone at least of the progression axes of the rotorcraft. The rate ofprogression of the rotorcraft that is taken into account may equallywell be a vertical speed, an air speed, an indicated air speed, and/or aground speed;

means for determining a slope of the rotorcraft;

barometric means for measuring the altitude of the rotorcraft;

radio altimeter means for measuring the height of the rotorcraft;

means for evaluating a deviation of the rotorcraft from a horizontalpath relative to a path setpoint previously acquired by the autopilot.The horizontal path of the rotorcraft is defined in particular togetherwith reference to the pitching axis and to the roll axis;

means for evaluating a deviation of the rotorcraft from a vertical path;

means for determining the progression of the rotorcraft by radionavigation; and

inertial sensor means, comprising accelerometers and/or angular speedsensors of the rotorcraft.

The performance of the path-maintaining function is preferablyassociated with an operation of displaying various kinds of data on adisplay member. The data that is displayed is selected to be appropriatefor enabling the pilot to make use of the path-maintaining functionunder the best possible conditions, particularly in an environment thatis hostile, uncertain, or poorly known.

More particularly, the displayed data comprises in particular theenvironment outside the rotorcraft, a model representing the rotorcraft,a representation of a landing zone for the rotorcraft moving relative tosaid landing zone, equally well approaching it conversely going awayfrom it, and the following illustrations relating to said outsideenvironment:

an illustration of a variation in the orientation of the rotorcraftrelative to a horizon line;

an illustration of a variation in the orientation and the position ofthe rotorcraft relative to the displayed landing zone; and

at least one illustration of information predicting the position and theorientation of the rotorcraft relative to the displayed landing zone.Said predictive information is derived in particular by projection fromvariation in the position and the orientation of the rotorcraft from atleast one current path setpoint.

The display operation is potentially performed equally well by a head-updisplay, by a head-down display, or by an in-between display.

The display of the environment outside the rotorcraft is preferably arepresentation of the world outside the rotorcraft as athree-dimensional image.

The illustration of a variation in the orientation of the rotorcraftrelative to the horizon line preferably comprises moving the model ofthe rotorcraft inclined in a lateral attitude relative to the horizonline that is displayed as being horizontally stationary and verticallymovable.

In a preferred implementation, the performance of the path-maintainingfunction depends on an operation of evaluating the pertinence of amovement of at least one said manual control member. Said evaluationoperation is an operation of using calculation to determine thepertinence of a movement of at least one manual control member, based onprocessing a signal for detecting such movement by calculation.

More particularly, the calculation processing of the detection signalcomprises an operation of smoothing the detection signal in time. On thebasis of the smoothed detection signal, an operation is performed ofdetermining the energy that has been developed in order to move themanual control member. Thereafter, an operation is performed ofcomparing said previously-determined developed energy with a predefinedthreshold that is considered as being representative of voluntarymovement of the manual control member by the person.

The method performed before performing said comparison is preferably asubsidiary operation of converting an analog signal that identifies saiddeveloped energy into a logic signal representative of the meandeveloped energy.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention are described with reference to thefigures of the accompanying sheets, in which:

FIG. 1 is a diagram showing an implementation of an autopilot inaccordance with a method of the present invention;

FIG. 2 is a diagram of display means associated with the autopilot shownin FIG. 1, for giving assistance to the pilot of a rotorcraftimplementing a method of the present invention; and

FIG. 3 is a diagram showing an example of an operation of evaluating thepertinence of a movement of a manual control member used by a person inan implementation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a rotorcraft 1 is an aircraft having rotors 2, 3 for movingwith six degrees of freedom in a local geographical frame of reference,on the basis of four progression axes taken into consideration forpiloting the aircraft. These progression axes comprise pitching P, rollR, yaw Y, and verticality V. In the implementation shown, the main rotor2 provides the rotorcraft 1 with lift and propulsion, and a tail rotor 3provides the rotorcraft with yaw guidance.

The rotorcraft 1 has manual control members 4 for modifying the behaviorin flight of the rotorcraft 1 via manual control linkages 5. The manualcontrol members 4 are moved by a person, in particular the pilot of therotorcraft, and they comprise in particular a collective pitch stick 6for varying the collective pitch of the blades of the main rotor 2, acyclic stick 7 for varying the cyclic pitch of the blades of the mainrotor 2, and rudder pedals 8 for varying the collective pitch of theblades of the tail rotor 3. A person moving the collective stick 6 makesit possible to modify the behavior of the rotorcraft 1 vertically V.Moving the cyclic stick 7 enables a person to modify the behavior of therotorcraft 1 in attitude P, R. Moving the rudder pedals 8 enables aperson to modify the behavior of the rotorcraft 1 in yaw Y.

The rotorcraft 1 is also fitted with an autopilot 9 that providespiloting assistance to the pilot of the rotorcraft 1 via automaticcontrol linkages 10. Conventionally, an autopilot 9 generates commandsin the application of previously-stored flight setpoints 11 in order tostabilize the rotorcraft 1 on the basis of those flight setpoints 11.The autopilot has memory means 12 for storing the flight setpoints 11relative to each of the progression axes P, R, V, Y of the rotorcraft 1.

Any deviation from maintaining a progression of the rotorcraft 1relative to the flight setpoints 11 is detected by the autopilot 9 bycomparing the previously-stored flight setpoints 11 with an actual stateof progression of the rotorcraft 1 as evaluated by means of onboardinstrumentation 13 of the rotorcraft 1.

On the basis of the flight setpoints 11, the autopilot 9 generatescommands that operate two superposed loops servo-controlling the flightparameters of the rotorcraft 1. In a stabilization mode 14, fastservo-control loops 15 act on correcting the attitude of the rotorcraft1. In higher modes 16, slow servo-control loops 17 serve to correctposition, speed, and/or acceleration of the rotorcraft 1 following acorrection of the attitude of the rotorcraft 1. Said slow and fastservo-control loops 15 and 17 enable the flight parameters of therotorcraft 1 to be varied relative to each of the progression axes P, R,V, Y of the rotorcraft 1.

The present invention proposes making use of the higher modes 16 thatare conventionally used for correcting the position, the speed, and/orthe acceleration of the rotorcraft 1 for the purpose of assisting thepilot of the rotorcraft 1 in maintaining a desired path selectivelyrelative to at least any one of the progression axes P, R, V, Y of therotorcraft.

In an approach of the present invention, such assistance takes accountof the rotorcraft pilot selecting a piloting assistance request relativeto at least one predetermined guide mode G (G1, G2, G3, G4, or evenmore). Said assistance request is more specifically a request 18 toacquire a path setpoint. The assistance provided to the rotorcraft pilotalso takes account of a person moving at least one of the manual controlmembers 4 in order:

to inhibit 19 the action of the autopilot 9 in the higher mode(s) 16relating to the progression axis(es) P, R, V, Y of the rotorcraft 1 onwhich the pilot of the rotorcraft 1 is taking action. Said highermode(s) 16 relate to a previously-selected guide mode G;

to replace 20 an initial path setpoint C (CV for a path setpoint inverticality V, CP for a path setpoint in pitching P, CR for a pathsetpoint in roll R, CY for a path setpoint in yaw Y) corresponding tosaid selected guide mode G and corresponding to the rotorcraft pilotacting on the manual control member 4 with a path setpoint Ccorresponding to the actual state of progression of the rotorcraft 1 asevaluated under the effect of the pilot of the rotorcraft interruptingmovement of the manual control member 4.

The flight setpoints 11 relating to the progression axes P, R, V, Y ofthe rotorcraft 1 that are not associated with the axes P, R, V, Yassociated with the previously-selected guide mode G remain unchanged.The commands generated by the autopilot 9 relating to all of theprogression axes P, R, V, Y that continue to be engaged are adapted bythe autopilot 9 depending on the current state of progression of therotorcraft 1. It should be considered that said progression axes P, R,V, Y that continue to be engaged are those relating to the higher modes16 that belong to a guide mode G that has not been subjected to a saidprior selection by the pilot of the rotorcraft 1, and consequently forthose progression axes P, R, V, Y that continue to be engaged, saidinhibition operation is without effect. The autopilot 9 adapts thecommands on the basis of said unchanged flight setpoints 11 and on thebasis of the path setpoint C that has been subjected to the acquisitionrequest 20.

Such provisions are particularly useful when the rotorcraft 1 isapproaching a landing zone in difficult situations. The pilot of therotorcraft 1 can make use of two capabilities of the higher modes 16 forassisting the pilot depending on the piloting needs of the rotorcraft 1,including in particular an ordinary and conventional need, and a need indifficult flight situations.

In a current assistance need, the pilot of the rotorcraft 1 may beassisted in flight by the higher modes 16 in a traditional mode ofstabilizing the rotorcraft 1. Such a current assistance need may be usedin a flight situation that is clear, remote from any obstacles, such asa cruising flight situation, or indeed a comfortable approach situation.By way of example, such a comfortable approach situation involves aknown approach zone that is clearly identified and that does not presentany particular difficulties in piloting the rotorcraft, and possiblyalso that has equipment on the ground for providing assistance inguiding the rotorcraft 1.

In a difficult flight situation, the pilot of the rotorcraft 1 may beassisted in flight by the higher modes 16, in particular during adifficult approach stage to an unknown landing zone and/or under weatherconditions that are difficult, or indeed in an environment that ishostile. In a difficult flight situation, the pilot of the rotorcraft 1can selectively change one or more flight setpoints 11 with reference toa guide mode G that has been selected by the pilot of the rotorcraft 1,by operating at least any one of the manual control members 4. The otherflight setpoints 11 relating to those guide modes G that have not beenselected are left unchanged and they are retained. The values of theflight parameters relating to said changed flight setpoints 11 arestored by the autopilot under the effect of the pilot of the rotorcraft1 interrupting movement of said at least one manual control member 4,with the commands relating to the other flight parameters being adaptedaccordingly by the autopilot 9.

In respective variant implementations of the present invention, a saidpiloting assistance request issued by the pilot of the rotorcraft 1 inaccordance with applying a path-maintaining function of the presentinvention either interrupts or does not interrupt the autopilot 9maintaining a basic mode of operation of the autopilot 9 that providesthe rotorcraft 1 with stability by damping variations in attitude bymeans of said fast servo-control loops 15.

More particularly, the pilot of the rotorcraft 1 has a multiple-choicemember 21 for activating the path-maintaining function on the basis ofselecting at least one guide mode G of the rotorcraft 1. The selectiveguide mode G is a guide mode G for which the pilot of the rotorcraft 1seeks to be able to intervene freely without being confronted by anypotential intervention of the higher mode(s) 16 relating to this guidemode G.

In addition, under the effect of an interruption of the intervention ofthe pilot of the rotorcraft 1 on the behavior of the rotorcraft 1, atleast one path setpoint C relating to the selected guide mode G is takeninto account by the autopilot 9 depending on the actual state ofprogression of the rotorcraft 1 as evaluated by the onboardinstrumentation 13 at the instant of said interruption.

Activating the multiple-choice member 21 generates an inhibit command 19for inhibiting the slow servo-control loop(s) 17 associated with thehigher modes 16 of guiding the rotorcraft 1, with respect to at leastone of the progression axes P, R, V, Y of the rotorcraft 1 (withverticality V serving for example to enable the rotorcraft to maintain aslope). Said at least one progression axis P, R, V, Y relates to said atleast one guide mode G as previously-selected by the pilot of therotorcraft 1. The pilot can control the rotorcraft 1 and modify theflight parameters of the rotorcraft 1 without encountering anyresistance that might potentially be generated by the autopilot 9.

The values of the flight parameters of the rotorcraft 1 relating to theother progression axes P, R, V, Y of the rotorcraft 1 (pitching P, rollR, and yaw Y for the same example of maintaining a slope), arepotentially adapted by the autopilot 9 on the basis of the flightsetpoints 11 as initially stored and that remain unchanged. Such anadaptation is performed depending on the current state of progression inflight of the rotorcraft 1, on the basis of the modification to thebehavior of the rotorcraft 1 made by the pilot of the rotorcraftrelative to the progression axis on which the pilot is intervening(verticality V for said example of causing the rotorcraft to maintain aslope) by means of the manual control member 4.

An interruption in the movement of the manual control member 4 by thepilot of the rotorcraft 1 generates an acquisition command 20 causingthe autopilot 9 to acquire the current path setpoint(s) C as evaluatedby means of the onboard instrumentation 13 concerning the actual stateof progression of the rotorcraft 1. The path setpoint(s) C concerned bythe acquisition command are those relating to the prior movement to themanual control member 4 by the pilot of the rotorcraft, with the otherflight setpoints 11 remaining unchanged.

The current value(s) of the flight parameter(s) as modified by saidmovement of the manual control member 4 by the pilot of the rotorcraft 1is/are acquired by the autopilot 9 to replace the initial value(s). Thecommands generated by the autopilot relating to the other flightparameters are corrected, where appropriate, relative to theirrespective initial values depending on the current state of progressionof the rotorcraft 1, which varies on the basis of the acquired pathsetpoint C corresponding to said value(s) of the flight parameter(s) asmodified by issuing the acquisition command.

It should be considered that the term “initial value” is used to meanthe value of a flight parameter as stored before the person moves themanual flight control member(s) 4, and providing the path-maintainingmode has been activated by using the multiple-choice member 21. Withrespect to implementing the method of the present invention, the ways inwhich said initial values of the flight parameters are acquired areimmaterial. For example, a said initial value may have been acquired ormodified by conventional means 40 for acquiring a flight setpoint of thekind commonly fitted to the rotorcraft, or it may have been acquiredfrom earlier activation of a guide mode G.

It should be observed that in the implementation shown, it is preferableto maintain stabilization assistance of the rotorcraft 1 by theautopilot 9 making use of the stabilization means 14, in spite of thepilot of the rotorcraft 1 activating the path-maintaining function thatmakes use of the higher modes 16. In a variant, such stabilizationassistance is inhibited under the effect of the pilot of the rotorcraft1 activating the path-maintaining function.

In FIG. 2, the assistance in maintaining the path of the rotorcraft bymaking use of the various guide modes is associated with visualassistance for the pilot of the rotorcraft showing the path beingfollowed by the rotorcraft relative to a landing zone.

Without being restrictive, it should be recalled that use of the methodof the present invention is particularly appropriate for a rotorcraftflying relative to a landing zone under conditions that are difficult,both with respect to the hostility of the external environment and withrespect to it being possible that the information relating of theterrain being approached is not certain.

The pilot of the rotorcraft has a display member 22 showing theenvironment 41 outside the rotorcraft 1, and preferably shownthree-dimensionally, with a representation of the relief surrounding thelanding zone 23 being approached by the rotorcraft 1. A model 24representing the rotorcraft 1 is shown, preferably diagrammatically inthe form of a line. This model 24 is moved by the autopilot 9 on thebasis of information supplied by the onboard instrumentation 13. Themovement of the model 24 is shown relative to a horizon line 25 that ishorizontally stationary but that moves vertically, and relative to saidlanding zone 23 that is being approached and that is showndiagrammatically.

The movement of the model 24 relates in particular to variation in theorientation of the rotorcraft 1 relative to the horizon line 25 andrelative to the landing zone 23 under approach, such as on the basis ofa variation in the lateral attitude tilt of the model 24. The horizonline 25 is held stationary horizontally, even when the rotorcraft turns,but it is movable vertically as a function of the longitudinal attitudebeing maintained by the rotorcraft.

A beam 26 projected from the model 24 shows the slope being maintainedby the rotorcraft relative to a predicted landing zone 27 of therotorcraft, given its current path. This beam 26 is associated with adiagrammatic illustration of a scale representing the value of a speedvector 29 representative of the speed of approach of the rotorcrafttowards the predictive landing zone 27.

On the basis of these indications, the pilot of the rotorcraft canquickly and easily correct the path of the rotorcraft by making use ofthe path-maintaining function proposed by the present invention.

It should be considered that the path-maintaining function may be usedboth during an approach to and when departing from a landing zone. Inaddition, the particularly appropriate purpose of the path-maintainingfunction for assisting in the guidance of a rotorcraft flying relativeto a landing zone does not in any way prejudice other applications thatmight be made of the path-maintaining function, including under variousflight conditions of the rotorcraft, e.g. including during cruisingflight.

In FIG. 3, it is proposed to validate a movement of any manual controlmember 4 that might be moved by a person in order to modify the flightbehavior of the rotorcraft by analyzing such movement. Such provisionsseek to avoid performing the path-maintaining function as a result ofmovements of the manual control member 4 that are not the result of itbeing moved voluntarily by the pilot of the rotorcraft. Validating thepertinence of a movement of the manual control member 4 by a person isperformed by analyzer means 30 for analyzing this pertinence on thebasis of determining the energy developed for moving the manual controlmember 4.

Said analyzer means 30 make use of detector means 31 for detecting thatthe manual control member 4 is being moved, and they generate adetection signal 32 representative of such movement. A smoothingoperation 33 is then performed to smooth the detection signal 32 overtime, and a smoothed detection signal 34 is generated.

From the smoothed detection signal 34, a developed energy signal 36 isdetermined by calculation 35 relating to the movement applied to themanual control member 4. The developed energy signal 36 is then compared37 with a predefined threshold 38 that is considered as beingrepresentative of a person deliberately moving the manual control member4. The developed energy signal 36 is preferably previously convertedinto a logic signal 39 representative of the mean developed energy, andon the basis of which said comparison 35 is performed.

What is claimed is:
 1. A method of using an automatic flight controlsystem for a rotorcraft having an autopilot that generates commands inpredefined operating modes, the commands causing a modification to thebehavior of the rotorcraft relative to at least one progression axis (P,R, V, Y), the rotorcraft having manual control members to cause amodification to the behavior of the rotorcraft relative to at least oneprogression axis (P, R, V, Y), the method comprising: performing by theautopilot a path-maintaining function including at least one guide mode(G) of the autopilot for guiding the rotorcraft relative to at least oneof the progression axes (P, R, V, Y) of the rotorcraft, in which thepath-maintaining function includes receiving an acquisition request fora person to acquire at least one path setpoint defined by at least oneflight parameter, each path setpoint relating to said at least one guidemode (G); as a result of the person driving at least one manual controlmember to cause a modification to the behavior of the rotorcraftrelative to at least one progression axis (P, R, V, Y), generating bythe autopilot: an inhibit command whereby the autopilot inhibits said atleast one guide mode (G) of the rotorcraft for which the acquisitionrequest has been issued and relating to said at least one progressionaxis relative to which the behavior of the rotorcraft is being modifiedby the person; and an acquisition command whereby the autopilot acquiresan acquired value of at least one flight parameter relating to each pathsetpoint to be acquired and applicable to at least one progression axisfrom which the behavior of the rotorcraft is being modified by theperson, the acquired value being acquired when said movement applied fordriving the manual control member is interrupted, and the acquired valuereplacing the value of the at least one flight parameter acquired by theautopilot prior to the acquisition command; the path setpoints relatingto the other guide modes (G) that have continued to be engaged and thatrelate to other progression axes (P, R, V, Y) being maintained in theirstates prior to said at least one manual control member being driven,and the commands generated by the autopilot relating to all of theprogression axes that continue to be engaged being adapted by theautopilot depending on the current state of progression of therotorcraft on the basis of said maintained path setpoints and on thebasis of the acquired value of the at least one flight parameter forwhich said acquisition command was issued.
 2. The method according toclaim 1, wherein the inhibit command for inhibiting said at least oneguide mode (G) of the rotorcraft is associated with a command formaintaining a basic mode of the autopilot, for stabilizing therotorcraft by damping its attitude.
 3. The method of claim 1, whereinthe predefined operating modes including at least one of: low speedoperating mode at air speeds of less than about 50 knots, approach stageoperating mode to an unknown landing zone, when close to the groundoperating mode, bad weather operating mode and hostile environmentoperating mode.
 4. The method according to claim 1, wherein theacquisition request comprises a selection operation wherein by amultiple choice control member, the person selects at least one saidguide mode (G); the automatic flight control system having a pluralityof guide modes (G) relating respectively to at least two of thefollowing: maintaining a rate of progression of the rotorcraft inattitude (P, R); maintaining a rate of progression of the rotorcraftvertically (V); maintaining a rate of progression of the rotorcraft inyaw (Y); maintaining an orientation of the rotorcraft relative to aheading; maintaining a slope of the rotorcraft; maintaining a hoveringposition; maintaining an acceleration of the rotorcraft relative to theaxes of the local geographical frame of reference; maintaining anacceleration of the rotorcraft relative to the progression axes (P, R,V, Y) of the rotorcraft; maintaining a coordinated turn by therotorcraft; maintaining the position of the rotorcraft relative to sideslip; maintaining an air speed of the rotorcraft; maintaining an angleof incidence of the rotorcraft; maintaining an altitude by therotorcraft; maintaining a height by the rotorcraft; and maintaining aspeed of the rotorcraft along a path.
 5. A method according to claim 1,wherein the acquisition by the autopilot of said path setpoint relatingto said acquisition command is an acquisition: either of a flightparameter relating to a variation in the collective pitch of the bladesof a main rotor of the rotorcraft, which parameter may be a constantvalue or a value variation to be maintained, the value of a flightparameter relating to a variation in the cyclic pitch of the main rotorand the value of a flight parameter relating to a variation in thecollective pitch of an anti-torque rotor of the rotorcraft remainingunchanged; or a flight parameter relating to a variation in the cyclicpitch of the blades of the main rotor, being either a constant value ora value variation to be maintained, the value of a flight parameterrelating to a variation in the collective pitch of the blades of themain rotor and the value of a flight parameter relating to a variationin the collective pitch of the anti-torque rotor remaining unchanged; orboth of said flight parameters relating to respectively to a collectivepitch variation and to a cyclic pitch variation of the blades of themain rotor, which may be constant values or value variations to bemaintained, the value of a flight parameter relating to a variation inthe collective pitch of the blades of the anti-torque rotor remainingunchanged; or else a flight parameter relating to a variation of thecollective pitch of the blades of the anti-torque rotor, which may be aconstant value or a value variation to be maintained, the value of aflight parameter relating to a variation in the cyclic pitch of the mainrotor and the value of a flight parameter relating to a variation in thecollective pitch of the main rotor remaining unchanged.
 6. The method ofclaim 1, wherein the value of said at least one flight parameter isevaluated depending on the actual state of progression of the rotorcraftas determined by onboard instrumentation of the rotorcraft; the onboardinstrumentation comprises a plurality of measurement means, including:means for determining a rate of progression of the rotorcraft along atleast any one of the progression axes (P, R, V, Y) of the rotorcraft,which rate of progression may equally well be vertical speed, air speed,indicated air speed, and/or ground speed; means for determining a slopeof the rotorcraft; barometric means for measuring the altitude of therotorcraft; radio altimeter means for measuring the height of therotorcraft; means for evaluating a deviation of the rotorcraft from ahorizontal path (P, R) relative to a path setpoint previously acquiredby the autopilot; means for evaluating a deviation of the rotorcraftfrom a vertical path (V); means for determining the progression of therotorcraft by radio navigation; and inertial sensor means, comprisingaccelerometers and/or angular speed sensors of the rotorcraft.
 7. Themethod of claim 1, wherein generating the acquisition command comprises:acquiring a current value of the flight parameter relating to each pathsetpoint to be acquired and applicable to the at least one progressionaxis from which the behavior of the rotorcraft is being modified by theperson; replacing the value of said flight parameter used by theautopilot prior to the manual control member being driven with saidcurrent value; using as a value of the setpoint path-maintaining a valueof said flight parameter evaluated depending on the actual state ofprogression of the rotorcraft and determined by onboard instrumentationof the rotorcraft.
 8. The method of claim 1, further comprisingperforming a display operation by a head-up display, by a head-downdisplay, or by an in-between display.
 9. The method of claim 8, whereinthe display of the environment outside the rotorcraft is arepresentation of the world outside the rotorcraft as athree-dimensional image.
 10. The method of claim 1, wherein operation ofthe path-maintaining function is dependent on a calculation operation todetermine the pertinence of the at least one manual control member beingmoved, the calculation being based on processing the detection signal.11. A method according to claim 10, wherein the inhibit command suspendsthe guide mode for which the acquisition request was issued.
 12. Anautomatic flight control system for a rotorcraft, the rotorcraft havingmanual control members to be driven by a person to cause a modificationto the behavior of the rotorcraft relative to at least one progressionaxe (P, R, V, Y) including pitching (P), roll (R), verticality (V), andyaw (Y), the automatic flight control system comprising: a memory forstoring flight setpoints relative to each of the progression axes;onboard instrumentation for evaluating an actual state of progression ofthe rotorcraft; and an autopilot that generates commands in predefinedoperating modes, the commands causing a modification to the behavior ofthe rotorcraft relative to at least one progression axis (P, R, V, Y);wherein the autopilot incorporates a multiple choice control member toselect at least one path-maintaining function including at least oneguide mode (G) for guiding the rotorcraft relative to at least one ofthe progression axes (P, R, V, Y); wherein the autopilot includes forthe path-maintaining function a detector for acquiring at least one pathsetpoint defined by at least one flight parameter and to detect drivingmovements of a manual control member for acquiring the path setpointwhen the manual control member is being driven by a person, the pathsetpoint relating to the guide mode (G) so that as a result of theperson driving the manual control member to cause a modification to thebehavior of the rotorcraft relative to one of the progression axes (P,R, V, Y), the autopilot generates: an inhibit command whereby theautopilot inhibits the guide mode (G) of the rotorcraft for which theacquisition has been issued and relating to the progression axisrelative to which the behavior of the rotorcraft is being modified bythe person; and an acquisition command whereby the autopilot acquires avalue of the flight parameter relating to the path setpoint to beacquired and applicable to the progression axis from which the behaviorof the rotorcraft is being modified by the person, the value of theflight parameter being acquired from the onboard instrumentationevaluating the actual state of progression of the rotorcraft, and uponthe driving of the manual control member being interrupted, an othervalue of the flight parameter takes the place of the value of the flightparameter that was acquired, the other value being derived from theflight setpoints stored in the memory prior to the inhibit command;wherein the autopilot maintains the values of the path setpointsrelating to the other guide modes (G) that have continued to be engagedand that relate to other progression axes (P, R, V, Y) prior to saidperson moving said manual control member, the commands generated by theautopilot relating to all of the progression axes that continue to beengaged being adapted by the autopilot depending on the current state ofprogression of the rotorcraft on the basis of said maintained pathsetpoints and on the basis of the value of the flight parameter forwhich said acquisition request was issued.
 13. The system of claim 12,wherein the onboard instrumentation comprises a plurality of measurementmeans, including: rate of progression determining means for determininga rate of progression of the rotorcraft along at least any one of theprogression axes (P, R, V, Y) of the rotorcraft, which rate ofprogression may be vertical speed, air speed, indicated air speed,and/or ground speed; slope determining means for determining a slope ofthe rotorcraft; barometric means for measuring the altitude of therotorcraft; radio altimeter means for measuring the height of therotorcraft; horizontal deviation evaluating means for evaluating adeviation of the rotorcraft from a horizontal path (P, R) relative to apath setpoint previously acquired by the autopilot; vertical deviationevaluating means for evaluating a deviation of the rotorcraft from avertical path (V); radio navigation progression means for determiningthe progression of the rotorcraft by radio navigation; and inertialsensor means, comprising at least one of: accelerometers, angular speedsensors.
 14. The system of claim 12, wherein the system further includesa display member for associating a performance of the path-maintainingfunction with an operation of displaying on the display member togetherwith a representation of the environment outside the rotorcraft, therepresentation having a model representing the rotorcraft, arepresentation of a landing zone for the rotorcraft moving relative tosaid landing zone, and the following illustrations relating to saidoutside environment: an illustration of a variation in the orientationof the rotorcraft relative to a horizon line; an illustration of avariation in the orientation and the position of the rotorcraft relativeto the displayed landing zone; and at least one illustration ofinformation predicting the position and the orientation of therotorcraft relative to the displayed landing zone, said predictiveinformation being derived by projection from variation in the positionand the orientation of the rotorcraft from at least one current pathsetpoint.
 15. A rotorcraft having an automatic flight control systemaccording to claim
 12. 16. The rotorcraft of claim 15, wherein therotorcraft further includes at least one display for a display operationof the rotorcraft during the predefined operating modes; the displaybeing at least one of: a head-up display, a head-down display, anin-between display.
 17. The rotorcraft of claim 16, wherein the displayof the environment outside the rotorcraft is a representation of theworld outside the rotorcraft as a three-dimensional image.